The invention described herein was made by an employee of the United States Government and may be manufactured and used by the government for governmental purposes without the payment of any royalties thereon or therefor.
1. Technical Field of the Invention
The present invention relates to the simultaneous measurement of two or more gases using optical path switching. More specifically, it relates to such measurement using dual beam spectroscopy, including gas filter correlation radiometry. It further relates to the balancing of optical intensities.
2. Discussion of the Related Art
Optical path switching has many potential applications, particularly in the field of dual beam spectroscopy. In dual beam spectroscopy, light from a radiation source traverses a measurement path and is then divided between two optical paths. Each optical path generally contains some medium through which the radiation is transmitted and thus partially absorbed and/or reflected. The key measurement in this type of spectroscopy is related to the intensity difference of the radiation that takes these two paths. For illustrative purposes, a gas filter correlation radiometer (GFCR), one example of a dual beam spectrometer, will be discussed in detail.
Gas filter correlation radiometers (GFCRs) may inter the concentration of a gas species along some measurement path either external or internal to the GFCR. In many GFCRs, gas sensing is accomplished by viewing alternately through two optical cells the emission/absorption of the gas molecules along the measurement path. These two optical cells, often called the correlation and vacuum cells, are an example of the media found in the two optical paths of a dual beam spectrometer. The correlation cell contains a high optical depth of gas species i and thus strongly absorbs radiation at the molecular transition wavelengths of the particular gas. In effect, the correlation cell acts as a spectral xe2x80x9cnotch filterxe2x80x9d to the incoming radiation, the spectral notches being coincident with the band structure of gas species i. The vacuum cell generally encloses a vacuum or a gas or gas mixture exhibiting negligible or no optical depth, e.g., nitrogen, an inert gas, or even clean dry air. The difference in signal between these two views of the emitting/absorbing gas species i within the spectral region of interest plus, or in combination with, the sum of the signals of these two views can be related to the concentration of this gas along the measurement path.
In one known GFCR for measuring a single gas concentration in a particular quantity disclosed in U.S. Pat. No. 5,128,797, issued to Sachse et al. and assigned to the National Aeronautics and Space Administration (NASA), the specification of which is hereby incorporated by reference, a non-mechanical optical path switch comprises a polarizer, polarization modulator and a polarization beam splitter. The polarizer polarizes light from a light source into a single, e.g., vertically polarized, component which is then rapidly modulated into alternate vertically and horizontally polarized components by a polarization modulator. The polarization modulator may be used in conjunction with an optical waveplate. The polarization modulated beam is then incident on a polarization beam splitter which transmits light of one orthogonal component, e.g., horizontally polarized, and reflects light of a perpendicular component, e.g., vertically polarized, In a gas filter correlation radiometer application, the transmitted horizontally polarized beam is reflected by a mirror, passes through a gas correlation cell, and is transmitted through a second beam splitter. The reflected vertically polarized beam passes through a vacuum cell, is reflected by a mirror and then reflected by the second beam splitter. The beam combiner recombines the horizontal and vertical components into a single beam which is read by a conventional detector. This approach has numerous advantages, such as no mechanical means being required to alternate the view of the detector through the correlation and vacuum cells, fast response, etc.
It would be desirable, in numerous applications, to be able to measure two or more gas concentrations simultaneously, either independently or non-independently, with a single device using an optical path switch. It further would be desirable to do such measurement with optimal optical and/or electronic balancing of optical intensities.
It is accordingly an object of the present invention to provide a device to simultaneously, but not independently, measure two or more gases of interest .
It is another object of the present invention to provide a device to simultaneously, but not independently, measure two or more gases of interest with negligible or no spectral interference.
It is another object of the present invention to provide a device to simultaneously and independently measure two or more gases.
It is another object of the present invention to provide a device to simultaneously and independently measure two or more gases with negligible or no spectral interference.
It is another object of the present invention to provide a device using an optical switch to simultaneously measure two or more gases for various applications requiring two optical analysis paths.
It is another object of the present invention to perform dual beam spectroscopy such as gas filter correlation radiometry using a single instrument to measure two or more gases in which the difference and sum signals can be obtained from only one detector for each gas wavelength region of interest.
It is another object of the present invention to accomplish simultaneous and independent measurement of two or more gases using a minimum of optical components.
It is another object of the present invention to accomplish simultaneous but not independent measurement of two or more gases using a minimum of optical components.
It is another object of the present invention to sense the total burden of a mixture of two or more gases using a single instrument.
It is another object of the present invention to detect some threshold level of the presence of any one or a combination of several gases using a single instrument.
It is still another object of the present invention to provide a device to simultaneously measure two or more gases of interest and optimize the balance of optical intensities.
It is still another object of the present invention to provide a device to simultaneously measure two or more gases of interest and optically optimize the balance of optical intensities.
It is a further object of the present invention to provide a device to simultaneously measure two or more gases of interest and electronically optimize the balance of optical intensities.
Additional objects and advantages of the present invention are apparent from the specification and drawings which follow.
The foregoing and additional objects are obtained by modulating a polarized light beam over a broadband of wavelengths between two alternating orthogonal polarization components. One orthogonal polarization component of the polarization modulated beam is directed along a first optical path and the other orthogonal polarization component is directed along a second optical path. At least one optical path contains one or more spectral discrimination means, with each spectral discrimination means having spectral absorption features of one or more gases of interest being measured. The two optical paths then intersect, and one orthogonal component of the intersected components is transmitted and the other orthogonal component is reflected. This forms a combined polarization modulated beam which contains the two orthogonal components in alternate order.
The combined polarization modulated beam is partitioned into one or more smaller spectral regions of interest where one or more gases of interest has an absorption band. The difference in intensity between the two orthogonal polarization components in each partitioned spectral region of interest is then determined as an indication of the spectral emission/absorption of the light beam along the measurement path. The spectral emission/absorption is indicative of the concentration of the one or more gases of interest in the measurement path.
More specifically, one embodiment of the present invention is a gas filter correlation radiometer which comprises a polarizer, a polarization modulator, a polarization beam splitter, a beam combiner, wavelength partitioning means and a detection means. The polarizer polarizes light from a light source into a single, e.g., vertically polarized, component which is then rapidly modulated into alternate vertically and horizontally polarized components by the polarization modulator. The polarization modulator may be used in conjunction with an optical waveplate. The polarization modulated beam is then incident on the polarization beam splitter which transmits light of one orthogonal component, e.g., horizontally polarized, and reflects light of a perpendicular component, e.g., vertically polarized. In a GFCR embodiment using two gas cells to measure two gases (hereinafter xe2x80x9ctwo gas/two gas cells embodimentxe2x80x9d), the reflected vertically polarized beam passes through a first gas correlation cell containing a first gas of interest, is reflected by a mirror and is then transmitted or reflected through the beam combiner. The transmitted horizontally polarized beam passes through a second gas correlation cell containing a second gas of interest, is reflected by a mirror, and is reflected or transmitted by the beam combiner. The beam combiner recombines the horizontal and vertical components into a single beam in which the polarization is time varying. The combined light energy is then partitioned into wavelength regions corresponding to each gasxe2x80x2 absorption band. A first optical bandpass filter transmits radiation centered on one gas band. This radiation is then focused on a first detector. Radiation reflected from the first optical bandpass filter is incident on a second optical bandpass filter. Radiation within the bandpass of the second filter, centered on the absorption band of the second gas, is transmitted and is focused on a second detector. Partitioning may be accomplished in a number of ways including the use of optical filters, gratings and prisms. Provided the first gas does not have absorption features within the spectral region defined by the bandpass filter of the second gas, the first gas correlation cell acts as a vacuum cell for the second gas, and vice versa. In some instances, the first and second gases, e.g., gases that do not chemically interact, may be contained within the same correlation cell. Measurements of both gases are accomplished simultaneously, independently and without interference. Furthermore, optical and/or electronic means are provided to balance optical intensities between the two optical paths.
Similar configurations are used for measuring three or more gases, including a GFCR embodiment which measures three gases using two gas cells (hereinafter xe2x80x9cthree gas/two gas cells embodimentxe2x80x9d) and a GFCR embodiment which measures three gases using three gas cells (hereinafter xe2x80x9cthree gas/three gas cellsxe2x80x9d embodiment). The presence of several gases can also be detected simultaneously but not independently, e.g., to sense the total burden of a mixture of two or more gases without needing to know the concentration of each individually or to detect some threshold level of the presence of any one or a combination of several gases.